1. Field of the Invention
The present general inventive concept relates to a bi-component type developing unit and an image forming apparatus employing the same, and more particularly, to a developing unit having a configuration to prevent a leak of developer and an image forming apparatus employing the same.
2. Description of the Related Art
In general, a bi-component type developing unit includes a developer made of a toner for forming an image and a carrier for carrying the toner by a magnetic force, a developing member which separates the toner from the developer and forms a toner image on a photosensitive medium, a regulation blade which regulates an amount of the supplied developer, and a sealing member which prevents a leak of the developer.
In addition, the developing member includes a magnetic cylinder provided with a plurality of magnetic poles and a sleeve rotatably installed on a circumference of the magnetic cylinder. Here, the magnetic poles include a catch pole which enables developer to be adsorbed onto a circumference of the sleeve, a main pole which contributes to developing, a returning pole which carries the carrier remaining on the circumference of the sleeve after contributing to a developing, and an exfoliation pole which separates the returned carrier from the surface of the sleeve.
FIG. 1 is a graph illustrating an interrelation of a magnetized width, a sleeve width, and a sealing width of a magnetic cylinder of a conventional developing unit, and FIG. 2 is a graph illustrating a distribution of a magnetic force in an axial direction of the magnetic cylinder with respect to magnetic intensities of 120 (mT), 130 (mT), and 140 (mT) of the conventional developing unit illustrated in FIG. 1.
Referring to FIG. 1, a magnetized width W1 of each of a plurality of magnetic poles 1 which form the magnetic cylinder is the same, and it is the same as a width W2 of a magnetic brush corresponding to an area of a sleeve 3 contributing to toner developing. A sealing member 5 is formed between end parts 3a and 3b of the sleeve 3 in a transverse direction and the width W2 of the magnetic brush to prevent a leak of the developer to opposite sides of the developing member. Also, a regulation blade 7 is formed to have a same width as that of the sleeve 3.
Meanwhile, if the developing unit is disposed as described above, combined magnetic force A or C at the opposite end parts of the sleeve 3 is larger than a magnetic force B at a center part thereof by an edge effect generated at opposite end parts of the magnetic cylinder as illustrated in FIG. 2. Accordingly, more developer is picked up at the opposite end parts of the developing member than at a center part of the developing member.
As described above, if the developer is increased at the opposite end parts of the developing member, a load to seal the developer at the sealing member provided at opposite end parts is increased, thereby causing the leak of the developer.
In addition, since an amount of the developer fed through the opposite end parts of the magnetic cylinder increases more than at the center part, the developer positioned in a developing gap at the opposite end parts of the magnetic cylinder is pressed and more stagnant than at the center part.
The amount of the carrier passing from the opposite end parts of the developing member to the photosensitive medium increases due to such stagnancy, and as a result, a total amount of the developer in the developing unit is decreased. In this case, an amount of a supplied developer picked up by an auger and supplied to the developing member decreases, thereby causing a non-uniform image, such as a mark. Also, toner scattering increases due to a decrease of an amount of the carrier charging the toner, to thereby contaminate an inside of the developing unit.
In addition, since the carrier carried to the photosensitive medium has a relatively larger volume in comparison with the toner, the carrier is caught in a cleaning blade of the photosensitive medium, or passes through the blade to be caught between the photosensitive medium and a charging roller, thereby accelerating scratch and abrasion of a surface of the photosensitive medium. Accordingly, as printing continues, the amount of the developer decreases, and a state incapable of performing printing due to damage of the surface of the photosensitive medium comes earlier, and accordingly, there arises a problem of reducing a lifetime of the developing unit and an image forming apparatus employing the same.